The present invention relates to the general technical field of machines for preparing hot beverages, and particularly coffee, in which the hot beverage is obtained by bringing a liquid into contact with aromatic grounds.
The present invention concerns, more specifically, an extraction cell, or unit, of the centrifugal type for a hot beverage preparation machine, preferably a coffee maker. The extraction cell includes a drum for retaining grounds, on which drum is mounted, in a fluid-type manner, by the intermediary of a sealing means, a cover provided with a liquid delivery orifice, the cell also including filtering means.
In the description which will follow, reference will be made particularly to coffee makers, which are presently the preferred class of machines in which the invention is to be employed. However, it should be readily apparent that the invention is applicable to all types of machines capable of preparing a hot drink by centrifuging, such as tea, for example.
In the preparation of coffee, and in particular for the preparation of espresso coffee, it is known that the consumer desires to obtain a coffee beverage having a certain quantity of foam at its surface. The consumer also desires that the quantity of foam obtained as a result of centrifugal filtration can remain at the surface of the coffee for a sufficiently long period of time. It is therefore also known that in coffee makers of the centrifugal type, it has been found to be extremely difficult to, at one and the same time, obtain a significant amount of foam and maintain that foam for a long period of time at the surface of the liquid. The phenomena involved in the creation of foam are presently insufficiently well understood to be completely mastered. It is simply known that the creation of foam is linked to the pressure resulting from the loss of charge, or pressure, of the liquid when it passes through the grounds, as well as the proportion of air mixed with the grounds and with the hot water in the filter, the proportion of foam obtained increasing with increases in pressure. In addition to the relatively poor understanding of the optimal conditions for obtaining foam by centrifuging, it has been found difficult to increase, in a significant manner by conventional means, the quantity of foam obtained in a centrifuge machine. In effect, a simple means frequently utilized until the present consisted precisely in increasing the operating pressure of the centrifuge machine in order to increase the quantity of foam produced. Such a pressure increase is obviously limited because of the attendant risk of degradation or rupture of parts resulting from a high pressure in the extraction cell and the filter. It is also in order to note that the possibilities of increasing the operating pressure are also connected in part with the capability of increasing the rotation speed of the extraction cell, such a speed increase also being limited because of mechanical considerations and having a negative influence on the total manufacturing cost of the apparatus.
In practice, centrifugal coffee machines thus present a number of limitations with regard to their possibility of furnishing significant quantities of long-lasting foam.
It has, however, already been proposed, in French Application No. 2487661, to provide a centrifugal coffee maker capable of assuring the production of foam at the surface of the coffee. In a conventional manner, the extraction cell is formed of a collecting drum on which is mounted, with the aid of latching means and sealing means, a cover defining, with the drum, a collection volume for the grounds and the hot water. Filtration of the water-grounds-air mixture is assured by a filtration means consisting of a series of radial grooves distributed around the periphery of the extraction cell and arranged between the cover and the drum. The centrifugal coffee maker includes, outside of the extraction cell, a deflecting wall against which the filtrate issuing from the radial grooves is projected at an acute angle. The combination of the projection angle, the speed of rotation and the action of the receiving wall is at the origin of the formation of a certain quantity of foam in the coffee.
If it can be considered that such a machine is capable of supplying coffee provided with foam, it is no less the case that its construction is relatively complex. In effect, it is necessary that the receiving wall and the drum be fabricated and disposed relative to one another with narrow dimensional tolerances in order to obtain a significant quantity of foam and to avoid pressure drops in the extraction cell. The fabrication of pieces having such close tolerances cannot be achieved without a significant increase in the total cost of the machine.
Moreover, it is already known, for example from French Patent No. 1584848, to reinforce the sealing of an extraction cell by mounting an annular sealing joint between the drum and the cover. The annular joint performs the classic functions assigned to a sealing joint without at the same time improving in a particularly observable manner the production of foam, the overall fluid tightness of such a system diminishing, in addition, significantly in the course of time as a result of wear experienced by the joint.